Abstract: Owing to the rapid development of urban underground space, the artificial freezing method has been widely used in the construction of subway-tunnel connecting channels. However, the mechanism of the freezing method as a joint result of water seepage and saline soil remains unclear. Hence, the originally planned freezing time is extended. A multifield coupling analysis under the interaction of water seepage and salt in the freezing process is proposed. The No.4 connection channel between Yongxing Avenue Station and Shennan Road Station of the Nantong Urban Rail Transit Line 1 is the research object in this study. First, based on heat transfer theory, as well as water seepage and salt migration in porous media, the physical-mathematical equation of the water-thermal-salt coupling method is established. Second, the finite element software COMSOL Multiphysics is used to simulate the freezing model test of saline soil mass, and the results of the model test are verified. Finally, the method above is applied to the construction simulation of the connecting-channel freezing method. A multiphysical field data monitoring equipment is designed, and the simulation and actual monitoring data are compared. The results show that the temperature of saline soil varies more slowly than that of non-saline soil. Additionally, a higher salt concentration in the soils implies a longer time required for their temperature to decrease to the same temperature. Moreover, the water head difference results in an asymmetrical frozen wall, and the thickness of the frozen wall upstream is smaller than that downstream. By calculating the thickness and average temperature of the frozen wall, it is discovered that the simulated freezing for 45 days does not satisfy the design requirements, and that the freezing time reaches 50 days. This study shows that safety is guaranteed when the freezing method is used in the connecting channel of Nantong Urban Rail Transit Line 1. The freezing method is a multifield coupling method that can be used under complex environmental conditions.

Key words: freezing technique, multifield coupling analysis, salt, water seepage, monitoring